1. Field of the Invention
The present invention relates to a laser scanning apparatus. More particularly, the present invention relates to a laser scanning apparatus employing a beam splitter, which is disposed between a polygonal mirror and an fθ lens.
2. Description of the Related Art
Laser scanning apparatuses, which are widely employed in printing devices, such as laser printers, form latent electrostatic images corresponding to images to be printed on the surface of a photosensitive medium by scanning laser beams on the photosensitive medium.
In general, a laser scanning apparatus has an optical system as shown in FIG. 1. Referring to FIG. 1, the optical system includes a laser diode 1, which emits a laser beam, a collimator lens 2, which has a slit 3 attached at the front thereof to collimate the laser beam emitted from the laser diode 1 so that it can be parallel to an optical axis, and a cylindrical lens 4, which focuses the collimated laser beam onto a reflective surface of a polygonal mirror 5 in a horizontally linear shape. The polygonal mirror 5, rotates horizontally at a constant speed and scans the laser beam passing through the cylindrical lens 4. A motor 6 rotates the polygonal mirror 5 at a constant speed. An fθ lens 7 has a refractive index with respect to an optical axis and polarizes a laser beam reflected from the polygonal mirror 5 in a main scanning direction and corrects aberration to focus the laser beam on a scanned surface. The optical system also includes a reflection mirror 8 which reflects a laser beam passing through the fθ lens 7 onto a surface of a photosensitive drum 9 in the form of dots. An optical sensor 11 receives laser beams reflected by a synchronous signal detecting mirror 10 and performs horizontal synchronization. When the polygonal mirror 5 rotates in a direction ‘A’ in FIG. 1, a laser beam is scanned in a direction ‘B’ in FIG. 1 so that image information can be recorded on the surface of the photosensitive drum 9.
To quickly print documents, the polygonal mirror 5 should be rotated at high speeds, or the number of reflective surfaces of the polygonal mirror 5 should be increased.
However, as the rotation speed of the polygonal mirror 5 increases, the motor 6 generates more noise. In addition, as the number of reflective surfaces of the polygonal mirror 5 increases, the size of the polygonal mirror 5 increases because each reflective surface of the polygonal mirror 5 has a uniform length in the main scanning direction.
Japanese Patent Publication No. 2001-183595, which is incorporated herein discloses a light scanning apparatus, which can reduce the size of a polygonal mirror even if the number of reflective surfaces increases, by reducing the length of each reflective surface of the polygonal mirror in a main scanning direction.
FIGS. 2 and 3 are a plan view and a side view, respectively, of the light scanning apparatus disclosed in Japanese Patent Publication No. 2001-183595. In FIGS. 1 through 3, the same reference numerals represent the same elements, and thus their descriptions will be omitted.
Referring to FIGS. 2 and 3, a laser beam emitted from a laser diode 1 and passing through a collimator lens 2, a slit 3, and a cylindrical lens 4 is reflected by a mirror 20 so that it is incident on an upper portion of the fθ lens 7. The laser beam passing through an fθ lens 7 is incident on a reflective surface of a polygonal mirror 5 and then reflected. The laser beam reflected from the polygonal mirror 5 passes through a lower portion of the fθ lens 7, is reflected by the reflection mirror 8, and is focused on a surface of a photosensitive drum 9. Therefore, as shown in FIG. 2, the laser beam emitted from the laser diode 1 is vertically incident on the polygonal mirror 5, and thus the polygonal mirror 5 can be designed so that the length of each reflective surface of the polygonal mirror 5 in a main scanning direction can be reduced. Therefore, the diameter of the polygonal mirror 5 can be reduced even if the number of reflective surfaces of the polygonal mirror 5 increases.
However, as shown in FIG. 3, the laser beam emitted from the laser diode 1 passes through the fθ lens 7 twice. Therefore, in order to obtain an image with a high quality, the shape of the fθ lens 7 in a sub scanning direction, requires a more complicated form. In other words, since the laser beam passes through the upper and lower portions of the fθ lens rather than a middle portion of the fθ lens 7, the fθ lens 7 should be precisely manufactured so that pitch errors can be prevented. In addition, in order to precisely manufacture the fθ lens 7, an effective length of the fθ lens 7 in a sub scanning direction should increase, which results in an undesirable increase in the total length of the fθ lens 7 in the sub scanning direction.
Seen from the sub scanning direction, the laser beam is slantingly incident on a reflective surface of the polygonal mirror 5 and then slantingly reflected from the polygonal mirror 5. Therefore, the length of the reflective surface of the polygonal mirror 5 in the sub scanning direction is increased.